ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus GmbHGöttingen, Germany10.5194/acp-13-5647-2013Estimate of surface direct radiative forcing of desert dust from atmospheric modulation of the aerosol optical depthdi SarraA.1FuàD.2MeloniD.11Laboratory for Earth Observations and Analyses, ENEA, Rome, Italy2Department of Physics, Sapienza University of Rome, Italy10062013131156475654This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from http://www.atmos-chem-phys.net/13/5647/2013/acp-13-5647-2013.htmlThe full text article is available as a PDF file from http://www.atmos-chem-phys.net/13/5647/2013/acp-13-5647-2013.pdf

Measurements carried out on the island of Lampedusa, in the central
Mediterranean, on 7 September 2005, show the occurrence of a quasi-periodic
oscillation of aerosol optical depth, column water vapour, and surface
irradiance in different spectral bands. The oscillation has a period of
about 13 min and is attributed to the propagation of a gravity wave able
to modify the vertical structure of the planetary boundary layer, as also
confirmed by satellite images. The wave occurred during a Saharan dust
event. The oscillation amplitude is about 0.1 for the aerosol optical depth,
and about 0.4 cm for the column water vapour. The modulation of the downward
surface irradiances is in opposition of phase with respect to aerosol
optical depth and water vapour column variations. The perturbation of the
downward irradiance produced by the aerosols is determined by comparing the
measured irradiances with estimated irradiances at a fixed value of the
aerosol optical depth, and by correcting for the effect of the water vapour
in the shortwave spectral range. The direct radiative forcing efficiency,
i.e., the radiative perturbation of the net surface irradiance produced by a
unit of optical depth aerosol layer, is determined at different solar zenith
angles as the slope of the irradiance perturbation versus the aerosol
optical depth. The estimated direct surface forcing efficiency at about
60° solar zenith angle is −(181 ± 17) W m<sup>−2</sup> in the
shortwave, and −(83 ± 7) W m<sup>−2</sup> in the photosynthetic spectral
range. The estimated daily average forcing efficiencies are of about −79 and
−46 W m<sup>−2</sup> for the shortwave and photosynthetic spectral range,
respectively.